This invention relates to wire ropes which are generally employed as travelling cables of elevators.
To aid in a full understanding of this invention, first, a known winding for wire rope will be described with reference to FIG. 1.
FIG. 1 shows a cross-sectional view of a construction for stranding wire rope, comprising; a fiber center core 1 made of a non-metallic material such as hemp or the like and a plurality of strands 2 which are twisted around the center core 1. Each of the strands 2 is made up of: a metal center wire 3 which is made of steel material and constitutes a part of wires forming the strand 2, a plurality of inner layer wires 4 of steel material which are laid around the metal center wire 3 and constitute a part of the wires, and a plurality of outer layer wires 5 of steel material which are laid around the inner layer wires 4 and constitute a part of the wires. The metal center core 3, the inner layer wires 4 and the outer layer wires 5 are made of the same steel material.
This wire rope when formed using conventional materials as set forth in accordance with FIG. 1 is unsatisfactory in terms of performance when it is subject to vibration. Its vibration damping effect is insufficient and a harmful resonance phenomenon takes place.
In an elevator with a hoisting unit having an induction motor, the speed is controlled by a thyristor circuit. In such a system, a high frequency vibration two to six times the power frequency takes place because of vibration induced by torque ripple of the motor accompanying the thyristor speed control. This vibration is transmitted through the travelling cables of the elevator to the cage thereof, which causes as uneasy motion in the ride and tends to irritate the passengers. More specifically, when the high frequency vibration is transmitted to the wire rope, a surging of the rope (hereinafter referred to as "rope surging" when applicable) occurs in association with the natural frequency of longitudinal vibration of the wire rope because the wire rope is a continuous unit, thereby resulting in the vibration of the cage. In addition, the magnitude of the noise level in the cage is increased by this rope surging. Hence, the conventional wire rope has a variety of unfavorable characteristics as described above.